They are using an editor only method of creating the texture that is skipped in packaged builds, I have a pull request up on github fixing that.
You can find it here if you are using a source version and want to merge it in: https://github.com/EpicGames/UnrealEngine/pull/5253
As for the material itself, it is part of the SteamVR plugins content folder and you can go edit it directly at will.